This invention relates to a method of producing high-purity metal members. More particularly, it relates to a method of producing members used for lining composite fuel cladding tubes in a nuclear reactor.
The fuel cladding tubes used in a nuclear reactor must have an excellent corrosion resistance, be non-reactive and conduct heat well, have high toughness and ductility, and have a small neutron absorption cross-section.
Zirconium alloys are widely used for fuel cladding tubes, because they meet these requirements.
Fuel cladding tubes made of a zirconium alloy can function very well under steady conditions, but when a great change takes place in load of a reactor there is the danger that they are subject to corrosion or stress cracking, and resultant breakage, because of the corrosive action of iodine gas released from the nuclear fuel pellets contained in the tubes, or the stresses generated by the expansion of nuclear fuel pellets.
In order to prevent such stresses or corrosion cracking in fuel cladding tubes, a barrier made of one of various metals is provided between each cladding tube and the nuclear fuel pellets therein. With cladding tubes made of a zirconium alloy, these tubes are lined with pure zirconium which acts as a metal barrier, which is disclosed in Japanese Patent Laid-Open Publication No. 54-59600/1979. This is because the pure zirconium lining is capable of remaining more flexible than zirconium alloys during neutron irradiation, and has the effect of reducing local strains produced in the zirconium alloy cladding tube to prevent stresses and corrosion cracking.
Experiments performed by the present inventors, however, have disclosed that the zirconium liner must be of an extremely high purity to maintain sufficient flexibility during neutron irradiation. In particular, when used under high-burning conditions, such a zirconium liner must have the purity of crystal-bar zirconium, particularly its low oxygen concentration, to produce the above effects. When the purity is of the sponge zirconium order, a liner can not provide the desired effects, because the degree of hardening due to irradiation is too high.
The crystal-bar zirconium can be obtained by iodinating sponge zirconium and subjecting the resulting iodide to chemical vapor deposition to form zirconium crystal bars. With this method, the reaction speed of the formation of zirconium by the thermal decomposition of zirconium iodide is extremely slow, and is therefore unsuitable for mass production. Thus, zirconium produced by this conventional method is very costly.
A vacuum arc furnace, a resistance-heating furnace, an electron-beam furnace, a plasma-arc furnace, or the like, is generally used for melting metals such as Zr, Ta, Nb, Ti, W, or Mo. The melting method which has the best refining effect is an electron-beam method in which the metal is melted in a high vacuum.
In the conventional electron-beam melting method, electron beams are applied to the metal material to melt it, and the molten metal which pools at the bottom of a crucible is drawn downward while being cooled. According to this method, low melting point impurity elements in the melt can be evaporated away, but impurities with low vapor pressures, such as oxygen, cannot be removed adequately.
Japanese Patent Laid-open Publication No. 56-67788 (1981) discloses a method of forming a nuclear fuel cladding liner by the electron-beam melting method. The publication describes, at page 3, left column, lines 19 and 20 and right column, lines 1 and 2, that a columnar ingot of 50 mm diameter, 500 mm length is formed by using a sponge Zr as a raw material and repeating electron beam melting of it twice in a vacuum atmosphere of 3.0.about.8.0.times.10.sup.-5 torr. From this description, it seems to use a rod melting method wherein members to be melted or a columnar ingot is disposed over a cavity and irradiated with electron beams to melt it, and the molten metal drops into the cavity thereby to form a purified columnar ingot. The rod melting method requires very great energy density to refine the sponge Zr.